For internal combustion engines, particularly diesel engines; increased efforts have been undertaken to provide measures for reducing emissions caused by exhaust gas components such as particulates and NOx. One method for addressing the above-mentioned issue is to equip the engine (such as an automobile—either by the original manufacturer or by aftermarket components) with an exhaust gas cleaning apparatus, such as a particulate filter or an exhausted gas cleaning catalyst (oxidative or reductive catalyst). However, if conventional lubricating oil compositions for internal combustion engines (which have a high sulfated ash content, a high phosphorus content and a high sulfur content) are used for these automobiles on which the exhaust gas cleaning apparatus is mounted, the soot produced by combustion of the lubricating oil in the engines would lead to deposits on the particular filter. Generally, the soot deposited oil the particulate filter is designed to be partially removed by oxidation or burning. However, there still is problem that residues produced by the burning the conventional oil soot, such as the metal oxide, the sulfated ash, and the carboxylate, may lead to plugging the filter.
Moreover, the high sulfur content contained in the fuel generates sulfuric acid or sulfates, which then migrate into the exhausted gas. The migration of the sulfuric acid or sulfate adversely effects to the catalyst cleaning effectiveness. In consideration of these problems, it is required to reduce the sulfur content in the fuel to a level as low as possible. Accordingly, it is anticipated that the reduction of sulfur content in fuel is accelerated in the near future. For example, the diesel fuel for automobiles on which a diesel engine is mounted now has a sulfur content so small as approximately 0.001 wt. %. In the case that the fuel contains a less amount of sulfur, the amount of a metal-containing detergent, which is incorporated into lubricating oil for neutralizing sulfuric acid, can be reduced. A lubricating oil works in engines for lubrication, but a portion of the lubricating oil is burned and exhausted. Therefore, it is considered that the amounts of phosphorus and metal in a lubricating oil preferably are as small as possible. Moreover, the reduction of the phosphorus content and sulfur content in the lubricating oil is preferred for reducing deterioration of the catalyst.
Japanese Provisional Patent Publication 2002-53888 (Patent Publication 1) describes a lubricating oil composition for internal combustion engines, which has a low ash content, a low phosphorus content, a low sulfur content, and a low chlorine content. The described lubricating oil composition gives less adverse influence to a particulate filter and an oxidation catalyst and shows good high temperature detergency, and therefore can satisfactorily cope with the anticipated exhausted gas regulations. The lubricating oil composition comprises a base oil (mineral oil and/or synthetic oil) having a sulfur content of 0.1 wt. % or less and the following additives dissolved or dispersed in the base oil in percent amounts based on the total amount of the oil composition:    (a) an ashless dispersant (alkenyl- or alkyl-succinimide or its derivative) in an amount of 0.01 to 0.3 wt. % in terms of the nitrogen content;    (b) a metal-containing detergent having a sulfur content of 3 wt. % or less and a TBN of 10-350 mg·KOH/g, in an amount of 0.1 to 1 wt. % in terms of a sulfated ash content;    (c) zinc dialkyldithiophosphate in an amount of 0.01 to 0.1 wt. % in terms of a phosphorus content; and    (d) an oxidation inhibiting phenol compound and/or an oxidation inhibiting amine compound in an amount of 0.01 to 5 wt. %.
The lubricating oil is further defined to have a sulfated ash content of 0.1 to 1 wt. %, a phosphorus content of 0.01 to 0.1 wt. %, a sulfur content of 0.01 to 0.3 wt. %, and a chlorine content of 40 ppm or less, and to contain an organic acid metal salt (which is contained in the detergent) in an amount of 0.2 to 7 wt. %.
Japanese Provisional Patent Publication 2003-336089 (Patent Publication 2) describes a lubricating oil composition comprising a base oil (mineral, oil and/or synthetic oil) having a lubricating viscosity and a sulfur content of 0.2 wt. % or less and the following additives dissolved or dispersed in the base oil in percent amounts based on the total amount of the oil composition:    (a) an ashless dispersant (alkenyl- or alkyl-succinimide or its derivative) in an amount of 0.01 to 0.3 wt. % in terms of the nitrogen content;    (b) a metal-containing detergent having a sulfur content of 3.5 wt. % or less and a TBN of 10-350 mg·KOH/g, in an amount of 0.1 to 1 wt. % in terms of a sulfated ash content;    (c) zinc dialkyldithiophosphate in an amount of 0.01 to 0.1 wt. % in terms of a phosphorus content;    (d) a phosphorus-containing ester selected from the group consisting of phosphoric acid esters, thiophosphoric acid esters, dithiophosphoric acid esters and phosphorous acid esters, in an amount of 0.002 to 0.05 wt. % in terms of a phosphorus content;    (e) an oxidation inhibitor selected from the group consisting of a phenol compound, an amine compound and a molybdenum compound, in an amount of 0.01 to 5 wt. %, wherein a ratio of the zinc dialkyldithiophosphate to the phosphorus-containing ester is 20/1 to 1/1 in terms of their phosphorus contents.
The lubricating oil is further defined to have a sulfated ash content of 0.1 to 1 wt. %, a phosphorus content of 0.01 to 0.1 wt. %, a sulfur content of 0.01 to 0.5 wt. %, and a chlorine content of 40 ppm or less, and to contain an organic acid metal salt (which is contained in the detergent) in an amount of 0.2 to 7 wt. %.